When a highway is laid out or ordered to be laid out across a railroad and the Commissioner of Transportation directs such highway to be carried over the railroad, he shall determine the length, width and material of the bridge over the railroad before the damages occasioned to any person by the taking of land for such highway are finally assessed; and said commissioner may require such bridge to extend beyond the railroad crossed by it. No structure shall be constructed or reconstructed over and across any railroad until the commissioner has determined the length, width, material and plan of such structure and its height above the roadbed of such railroad and the necessity for such construction or reconstruction.